This invention relates to an electro-photography type image forming apparatus capable of suppressing energy consumption.
Conventionally, image forming apparatuses of the electro-photography type have been constructed in such a manner that an electrostatic latent image formed on a surface of a photosensitive member in an imaging assembly is developed into a toner image by electrically attracting toner particles to the latent image, the thus formed toner image is transferred onto a copy sheet which is fed from a sheet storage portion to an image exposure position on the surface of the photosensitive member and fixed in a fixing unit, and then the copy sheet carrying the fixed toner image is discharged outside of the image forming apparatus.
In such an image forming apparatus, a so-called thermal fusion and fixing method has been adopted in the fixing unit for the purpose of high speed image fixation and safe operation. Specifically, according to the thermal fusion and fixing method, the fixing unit includes a pair of rollers including a heater roller internally provided with a heater and a presser roller. The presser roller is pressed against the heater roller with a specified pressure. While a copy sheet carrying a transferred toner image is transported between the heater roller and the presser roller, the toner image is fused by heat of the heater roller which is heated at a predetermined temperature (e.g., about 200.degree. C.) for image fixation by the heater (or merely fixing temperature), while being pressed against the copy sheet with the specified pressure, thereby fixing the toner image onto the copy sheet.
In order to prevent a fluctuation of temperature, the heater of the heater roller has a large heat capacity and thus consumes a great amount of power.
The image forming apparatus is provided with a fan in the proximity of the fixing unit for cooling peripheral devices (suppressing rise of the temperature) arranged in the vicinity of the fixing unit. By dispersing heated air in the vicinity of the fixing unit with the use of the fan and exhausting the heated air to outside of the image forming apparatus through an escaping hole formed in the apparatus main body, rise of the temperature of the peripheral devices is suppressed.
It is often the case that the image forming apparatus is put into a stand-by mode where copying is not performed, after a main switch for the apparatus is turned on to supply power thereto, which results in a waste of energy. To suppress energy consumption of the image forming apparatus during the stand-by mode, the image forming apparatus is provided with energy saving mode which is operated when a predetermined time duration lapses after the apparatus enters a stand-by mode.
When the apparatus is set to the energy saving mode, heating operation of the heater in the heater roller is suspended (or energization of the heater power supply is leveled down) to put the heater into a pre-heated state in which a surface temperature of the heater roller is maintained at a level lower than the fixing temperature. At the time of suspending driving of the heater or leveling down of the heater power supply, operation of the fan is simultaneously suspended (halted). At this state, a message indicative that the image forming apparatus is in the energy saving mode is displayed.
The above conventional image forming apparatus has the following problems. As mentioned above, when the image forming apparatus is set to energy saving mode, leveling down of the heater power supply is conducted simultaneously with suspending the operation of the fan. In such case, even after the heater power supply is leveled down when the image forming apparatus is set to the energy saving mode, the heater, which has a large heat capacity as previously mentioned, still radiates heat, and such radiant heat is transmitted to the peripheral devices of the fixing unit, because the radiant heat is no longer dispersed and exhausted by the operation of the fan. The thus transmitted radiant heat at some point in time raises the temperature of the peripheral devices arranged near the fixing unit, thereby adversely affecting the peripheral devices, in particular, causing adverse effects to the devices made of synthetic resin which are subjected to heat deformation and thermal fatigue.